1. Field of the Invention
The invention relates to a pixel driving circuit and, in particular, to a pixel driving circuit compensating threshold voltage and power supply.
2. Description of the Related Art
Organic light emitting diode (OLED) displays that use organic compounds as a lighting material to light are flat displays. The advantage of the OLED displays is small size, light weight, wider viewing angle, high contrast ratio and high speed.
Active matrix organic light emitting diode (AMOLED) displays are currently emerging as the next generation of flat panel displays. Compared with active matrix liquid crystal displays (AMLCD), the AMOLED display has many advantages, such as higher contrast ratio, wider viewing angle, thinner module without backlight, low power consumption, and low cost. Unlike the AMLCD display, which is driven by a voltage source, an AMOLED display requires a current source to drive a display device EL (electroluminescent). The brightness of display device EL is proportional to the current conducted thereby. Variations in current level have a great impact on brightness uniformity of an AMOLED display. Thus, the quality of a pixel driving circuit is critical to the quality of an AMOLED display.
FIG. 1 shows a conventional 2T1C (2 transistors and 1 capacitor) pixel driving circuit 10 in an AMOLED display. Pixel driving circuit 10 comprises transistors Mx and My. When signal SCAN turns on transistor Mx, data signal shown as Vdata in the FIG. 1 is loaded into a gate of p-type transistor My and stored in capacitor Cst. Thus, there will be a constant current driving display device EL to emit light. Typically, in an AMOLED display, a current source is implemented by a P-type TFT (My in FIG. 1) gated by data signal Vdata and having source and drain connected to Vdd and the anode of display device EL, respectively, as shown in FIG. 1. The brightness of display device EL with respect to Vdata therefore has the following relation.Brightness∝current∝(Vdd−Vdata−Vth)2 
Where Vth is a threshold voltage of transistor My and Vdd is a power supply voltage. Since there is typically a variation in Vth for a LTPS type TFT due to a low temperature polysilicon (LTPS) process, it is supposed that a non-uniformity problem in brightness exists in an AMOLED display if Vth is not properly compensated. Moreover, a voltage drop in the power line also causes the brightness non-uniformity problem. To overcome such problems, implementation of a pixel driving circuit with threshold voltage Vth and power supply voltage Vdd compensation to improve display uniformity is required.